1. Field of the Invention
The invention relates to a rotary anode for X-ray tubes comprised of a dense carbon coating and the method of manufacturing such anodes. Dense-coating of anode surfaces with carbon is achieved by depositing a carbon coating generated by decomposition of hydrocarbon gases at temperatures below the thermal decomposition temperature on rotary anode surfaces.
2. The Prior Art
Modern X-ray diagnostic procedures such as computer tomography require rotary anodes with a very high thermal capacity In addition to having high thermal capacity, these rotary anodes must also be capable of withstanding acceleration to speeds as high as 10,000 r.p.m followed by sudden braking.
Conventional rotary anodes consist of high melting metals such as tungsten, molybdenum or related alloys. Such materials, however, have a very high specific weight, precluding their use in rotary anodes employed in computer tomography where sudden high speed acceleration and braking is required.
Graphite, because of its low specific weight, has been found to be a suitable material for the manufacture of these rotary anodes. Furthermore, graphite has a significantly higher heat capacity and superior thermal emissivity (heat radiation) when compared to metals which melt at high temperatures
There are presently two conventional designs for graphite anodes. In one variety, the basic body of the rotary anode is made from graphite provided with a thin target layer of a high melting-temperature metal, such as a tungsten-rhenium alloy in the path of radiation. This target layer is preferably applied by coating methods known in the art such as the CVD-procedure.
In the other conventional rotary graphite anode design, the target layer is made from a high melting temperature metal such as molybdenum or a molybdenum alloy which is joined outside of the target layer with one or more sintered graphite sections, preferably by soldering.
These conventional rotary graphite anodes suffer from the disadvantage that fine graphite particles can become embedded in the porous graphite surface as a result of abrasion. These embedded fine graphite particles cannot be adequately removed by cleaning methods such as ultrasound bath cleaning.
The embedded fine graphite particles also lead to "dusting"; i.e., fine graphite particles become detached from the surface of the graphite by the action of electrostatic forces or by centrifugal forces and deposit within the X-ray tube This may lead to flash-overs in the X-ray tube, Particularly at voltages in excess of 100 kV.
Furthermore, gases which had been absorbed by the graphite due to its high porosity (normally about 20% of the volume) may be released in the vacuum of the X-ray tube, causing a deterioration of the vacuum and, in turn, operating interferences in the form of flash-overs.
To avoid "dusting", and the release of gas from rotary graphite anodes graphite surfaces are sealed with coating materials. German published patent disclosure DE-OS 31 34 196 describes the coating of a rotary graphite anode with a coating consisting of pyrolytic carbon formed by decomposing a gaseous hydrocarbon compound at temperatures in the range of 1000.degree. to 1100.degree. C. This coating significantly increases the high-voltage stability; however, the thermal emissivity is significantly impaired compared to rotary anodes with an uncoated graphite surface, as is the ability of the anode to withstand loading.
Another drawback of a pyrolytically applied carbon coating is the fact that a high coating temperature of about 1000.degree. C. or higher is required at pressures ranging from about 10 to about 1000 mbar.
Rotary graphite anodes in which one or more graphite parts are joined with a basic body of high-melting metal by soldering require that the graphite coating be applied prior to joining the parts with the basic body by soldering. At high coating temperatures, hydrogen released in the course of the coating step leads to embrittlement of the solder and, consequently, damages the composite material.
Another more serious drawback of pyrolytically deposited carbon coatings employed with sintered graphite is that the different coefficients of expansion of sintered graphite and pyrolytic carbon coating causes high stress in the coating, which negatively impacts on the resistance of the coating to thermal shock. Adhesive or cohesive failure due to mechanical action occurs in the presence of prestresses in the coating even under lower loads.